The present invention concerns an amplifier circuit for receiving an optical signal, which amplifier circuit comprises:                a light sensitive member arranged to receive an optical input signal and to deliver an electric signal in response to the received optical signal, and        an amplifier unit with a first input arranged to receive the electric signal from the light sensitive member, wherein the amplifier unit is arranged to influence the amplification of the electric signal and to deliver an amplified output signal via a first output, wherein the amplifier unit does not have any further input specially intended for controlling the amplification of the amplifier unit.        
The invention also concerns a communication system and a method of controlling the amplification in an optical input stage.
The above described amplifier circuit may suitably be used for amplifying an input signal originating from an optical fibre. The amplifier circuit may thus constitute a fibre optic input stage. Such an input stage is used in optical communication systems. Such an input stage may be followed by further amplifiers. The input stage thus often ensures that an input signal from an optical communication path is adapted to an input of a subsequent amplifier.
FIG. 1 shows schematically an example of such an optical input stage according to the prior art. The input stage comprises a light sensitive member 301. The light sensitive member 301 may for example constitute a photo-diode. The light sensitive member 301 delivers an electric signal in response to an optical input signal, for example from an optical fibre (not shown in the figure). According to the shown example, the cathode of the photo-diode 301 is connected to a bias voltage V1. The circuit comprises an amplifier unit 302, which is often called preamplifier. The amplifier unit 302 has a first input 304 which receives an electric signal from the light sensitive member 301. The amplifier unit 302 influences the amplification of the electric signal and delivers an amplified output signal via a first output 306. The shown photo-diode 301 delivers a current into the input 304, wherein the strength of the current depends on detected light. The amplifier unit 302 converts the current to a voltage at the output 306. The transfer function therefore gets the unit V/A, i.e. ohm. The amplification of the amplifier unit 302 may thus be stated in ohm. The amplifier unit 302 may also comprise an internal amplification control unit 308 which for example may be arranged to reduce the amplification at too high currents.
The English abstract of JP-A-10284955 shows an example of this kind of optical input stage. This document shows such an input stage with a control circuit for controlling the amplification in response to an average value of the optical input power.
Also WO99/28768 shows an optical input stage where a control circuit controls a variable impedance element in the form of a diode connected to an amplifier input.
The English abstract of JP-A-09298426 shows an optical input stage with a preamplifier. In this case the preamplifier has a special input where a control signal may be connected for controlling the amplification of the preamplifier. However, an optical input stage usually lacks a special input for controlling the amplification. An example of an input stage is the one which is sold with the name MC2006 of the fabrication Microcosm. For example this input stage comprises an internal control unit for reducing the amplification at too high currents. However, the input stage lacks a special input for being able to control the amplification.